I used Mplus to test a moderated mediation model, in which only first stage moderation is hypothesized. X is predictor, Y is outcome variable, M is mediator and W is moderator. I labled coefficients (B1-B4) when the model is specified. However, I get error information that " Unknown variable(s) in an ON statement: (B1)".I guess I might encounter similar warnings with B2-B4 too. Here is my command: